If you are not already a subscriber to Cook's Illustrated Magazine (which I imagine to be very unlikely), I highly recommend your subscribing to the digital version because you have access to all the old issues and will have lots of fun checking the site out.)
This is a very loose adaptation of a recipe from Cook's Illustrated magazine. This is really, really good; The biggest problem is these margaritas do go down very easily.
Adapted from the July-August 2000 Issue of Cook's Illustrated Magazine
4 limes, juiced
2 lemons, juiced
1 small orange, juiced
1 cup Reposado tequila (I use Hornitos)
1 cup Triple Sec (I use Bols and never substitute Cointreau)
Glug of Rose's Lime Juice (optional)
Mix all of the above in a pitcher. Add simple syrup* to taste; the amount you add will depend on how tart the fruit is and on how sweet you like your drink. Remember, the Triple Sec is sweet. At this point, taste a little to make sure it's sweet enough - but not too sweet because you can always add more simple syrup until you get it right. Keep in mind the Rose's Lime has sugar in it too even though it imparts that sweet/sour taste.
In batches (because it won't all fit), shake the above mixture from the pitcher in a metal cocktail shaker filled with ice until the shaker is frosty. Serve over ice cubes in old-fashioned glasses or large wine glasses (my choice is the Reidel Ouverture Magnum). Personally, I do not rim the glasses with salt and never use margarita glasses, but if you must, feel free.
Bring equal amounts of sugar and water to a boil, just until the sugar dissolves. Will keep in a jar in the refrigerator. It's good to have on hand in the summer for iced tea and lemonade.